With regard to biodegradable macromolecular substances, various resins such as polylactic acid, polybutylene succinate, polyethylene succinate, polycaprolactone and polyhydroxybutyrate have been proposed already.
Those biodegradable macromolecular substances have a characteristic that they have a biodegradable property but, as compared with resins such as polyolefin, aromatic polyester resin and engineering plastic, there is a problem that their physical property, particularly mechanical strength, is not so sufficiently good for practical use. In order to solve such a problem, resin where a biodegradable polyester resin is copolymerized with polyether such as polyethylene glycol has been proposed (refer to Patent Document 1).
In the copolymer proposed therein however, no sufficient molecular weight is available and its mechanical physical property is not sufficient for practical use. Therefore, polymers having a biodegradable property and a sufficient mechanical physical property have been demanded.
In the meanwhile, a method where two or more aliphatic polyesters are blended so as to improve the mechanical property has been attempted as well. For example, polybutylene succinate has a sufficient elongation but, in some uses, strength such as elasticity is somewhat low and is too soft while, although polylactic acid has a high rigidity, it lacks in elongation and has a hard and fragile property whereby its use is limited. Therefore, compositions where they are subjected to a melt blending have been proposed (refer to Patent Document 2).
However, although the compositions disclosed as such are compositions which are suitable for the formation of film and the like, they are insufficient in terms of physical property for the field of use as injection molded products where higher impact strength and rigidity are demanded or, to be more specific, interior parts for automobiles, home appliance parts and baskets whereby there has been a demand for a composition having both higher impact strength and rigidity.
[Patent Document 1]
JP-A-8-59808
[Patent Document 2]
JP-A-9-272789
An object of the present invention is to provide an aliphatic polyester polyether copolymer having a biodegradable property and a sufficient mechanical physical property, a process for producing the same and an aliphatic polyester composition having both higher impact strength and rigidity.